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Natural Selection
In 1831, Charles Darwin left England on a ship for a five-year voyage around the world.
This voyage would cause people to change forever the way they looked at the natural
world. The observations that Darwin made during his voyage led him to think that life on
Earth had changed over its long history. In other words, Darwin had gathered evidence
that supported evolution. In biology, evolution is the process through which species
change over time.
Darwin’s observations on his voyage also helped him form the Theory of Natural
Selection to explain how evolution occurs. The Theory of Natural Selection proposes
that certain individuals are better adapted to their environment. These individuals have a
better chance of survival and therefore of reproducing. As a result, these individuals are
more likely to pass on their genes to the next generation.
Darwin’s Theory of Natural Selection is based on four key principles: overproduction,
inherited variation, competition, and selection.

Overproduction: A population produces more offspring than can possible survive.
Most populations have more offspring each year than local resources can support
leading to a struggle for resources. Animals and plants have great reproductive
potential, but most offspring are lost to predators, disease, and other factors.
Relatively few survive to reproduce.

Inherited Variation: Each individual in a population has its own set of traits.
Darwin knew that offspring inherit traits from their parents. However, he did not
know how heredity operated. Unfortunately, Darwin was unaware of the work of
Gregor Mendel. Had Darwin known what Mendel had discovered about heredity, he
would have been able to explain the inherited variations he observed among the
individuals of a population. Inherited variations are differences in the traits of
individuals of the same species. Although Darwin did not understand the basis for
inherited variations, he did recognize their importance. Darwin thought that such
variations were crucial for survival.

Competition: There is competition among the individuals within a population. Darwin
reasoned that because resources are limited, all individuals within a population must
struggle to survive. They must struggle for food, shelter, and water. They must
also struggle to avoid diseases and predators. For many individuals in the
population, this struggle is futile, and these individuals die. Only those that prove
successful in their struggle survive.

Selection: Those individuals that win the struggle for survival will be able to
reproduce. Darwin reasoned that those individual who win the struggle have the
traits that are needed to survive. Such traits are known as adaptations. An
adaptation is a trait that increases an organism’s chances of survival in its
environment. The offspring of individuals with these favorable traits will, in turn,
be more likely to survive and reproduce more individuals with the trait. After many
generations, more members of the species will have the favorable trait.
Darwin recognized the importance of adaptations when he closely examined a group of
birds known as finches. Darwin had seen a variety of finches on the Galapagos Islands, one
of the many stops that his ship made on its long voyage. Actually, Darwin did not recognize
the importance of variations in these birds until he had returned to England, There he
took his time in taking a much closer look at the finches, especially their beaks. There
turns out to be 14 species of finches on the Galapagos Islands. One species survives by
eating the seeds produced by trees. Three species survive be eating seeds from the
ground. Three other species survive by eating seeds from cactus plants. The last seven
species survive by eating insects that live in trees. The beak of each species of finches is
adapted to obtaining the food in its environment. For example, imagine that some finches
with beaks adapted to getting seeds from cactus wind up in a forest. These birds do not
have the adaptations to get seeds or insects from trees. As a result, they are not likely to
win the struggle for survival in their competition against those finches that do have the
necessary adaptations. Those that are best adapted are most likely to survive and
produce offspring.
Darwin returned from his voyage in 1836. However, he did not publish his Theory of
Natural Selection until 1859. Why did he wait almost 25 years before publishing his
Theory of Natural Selection? Darwin recognized the impact that his theory would have on
the public. Many people would be reluctant to accept his conclusion that life evolves. Even
more people would be reluctant to accept Darwin’s conclusion that evolution can result in
the production of a new species. In 1859, Darwin finally published his book called On the
Origin of Species by Means of Natural Selection.
Models of Evolution:
The fossil record shows that evolution can be an extremely slow process. Scientists
call this slow process gradualism. Gradualism states that a species changes very slowly
over a very long period of time. The fossil record is also used as evidence that evolution
does not always happen slowly, as in gradualism. Rather, evolution can occur in sudden
spurts or rapidly, this process of evolution is called punctuated equilibrium. In this model,
a species remains unchanged for long periods. This period of little or no change is known
as stasis. A species may them suddenly undergo a change that occurs rather rapidly.
a)
Gradualism
b)
Punctuated Equilibrium
Name ____________________________________
Period ___________
Natural Selection
1.
Define evolution?
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2. Which individuals in a population are more likely to pass their genes on to the next
generation?
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3. What are the four principles on which the theory of natural selection is based?
a ._____________________
c. _____________________
b. _____________________
d. _____________________
4. What is meant by overproduction by a population?
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5. What is inherited variation?
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6. What did Darwin mean when he said that individuals struggle to survive?
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7. How are adaptations important to natural selection?
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8. How does variation in the Galapagos finches support the theory of natural
selection?
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9. The struggle to survive is sometimes referred to as “survival of the fittest”. What
does the term fittest mean in this context??
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10. Define gradualism?
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11. What is punctuated equilibrium?
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